This invention relates to a pedicle attachment assembly for use in conjunction with a rod for immobilising bone segments, particularly in the spine, so as to allow the segments to fuse together.
To achieve bone fusion or arthrodesis a stable environment has to be created. The pedicle attachment and rod assembly immobilises the bony junction so that bone graft of bone substitute material which is placed across the junction can consolidate. This consolidation process takes place during the healing process and creates a bony bridge or solid fusion mass across and between the bony junction. The process of arthrodesis has been well documented in the field of batch mechanics and traumatology.
According to a first aspect of the present invention there is provided a pedicle attachment assembly comprising a pedicle attachment device having a head, a polyaxial housing having a through bore and comprising a base portion and two posts upstanding from the base portion, the posts defining therebetween a slot for receiving a rod, a saddle-shaped element disposed or disposable in the slot and having a part cylindrical recess in an upper surface for engagement in use with the rod, the head of the attachment device and the lower end of the through bore having interengageable first portions and the head of the attachment device and the saddle-shaped element have interengageable second portions, the first portions and the second portions being shaped to allow the attachment device to pivot relative to the housing and the saddle-shaped element, and a cap assembly for use in clamping the rod in the housing.
According to a second aspect of the invention there is provided a pedicle screw assembly comprising a pedicle screw having a threaded stem and a head of larger diameter than the stem, a polyaxial housing having a through bore and comprising a base portion and two posts upstanding from the base portion, the posts defining therebetween a slot for receiving a rod, a saddle-shaped element disposed or disposable in the slot and having a part cylindrical recess in an upper surface for engagement in use with the rod, the lower surface of the head of the screw and the lower end of the through bore in the housing having complementary part spherical or substantially part spherical surfaces for engagement with one another, and a cap assembly for in use clamping the rod in the housing, the cap assembly comprising a cap having an internally threaded skirt portion for threadably engaging external threads provided on the two posts and a top portion having an internally threaded aperture, and a set screw engageable in the internally threaded aperture so that diametrically opposed regions of the lower edge of the cap and the set screw in use provide three point clamping between the rod and the cap assembly.
The invention will now be more particularly described, by way of example, with reference to the accompanying drawings.